The present invention relates to a wet-chemical metallization process for electrically non-conductive, previously chemically activated plastic substrate sheets, and more particularly, to a wet-chemical metallization process for sheets of nonwoven, needlefelt or open-pore foamed materials, in which process the chemically activated substrate sheets are brought into contact with a metallizing solution, and after the chemical metallization has been carried out, the metallizing solution is again separated from the substrate sheets by the influence of gravity or centrifugal force, and in which the metallized substrate sheets are then rinsed with rinsing water and the adhering residues of metallizing solution are removed.
Chemical metallization of plastic sheets, e.g. made of nonwoven, needlefelt or open-pore foamed materials, is nowadays carried out on a large scale. Such sheets made of textile material are, among other things, inexpensive to produce and readily variable in terms of their thickness and porosity. Application of a metal layer to the surface of the sheets achieves additional beneficial properties in these products such as, e.g., electroconductivity, magnetic and heat-conductive properties. In order to metallize the plastic surfaces, such surfaces must first be prepared for metal deposition. In many cases, the plastic surfaces are first pretreated mechanically (roughening) or chemically (etching).
As the plastic fibers made up into the sheets are electrically insulating and thus unsuitable for direct chemical metal deposition they must first be "activated", i.e. noble metal-containing catalytically active particles are deposited on the plastic surface. These particles, which must be deposited on the plastic surface so as to adhere as strongly as possible, serve to catalyze the subsequent, chemical, metal deposition proper from a metastable solution. The preferred catalyst solutions are prepared on the basis of palladium/tin. In principle, however, other noble metal-containing compounds are also usable, provided they initiate the kinetically inhibited chemical reduction of dissolved metal ions on the plastic surface by a reductant also present in the solution.
Once the plastic surface has been suitably pretreated and activated, chemical metallization is carried out after any intermediate steps still to be carried out such as hydrolysis or acceleration. The metals predominantly deposited on plastic surfaces on an industrial scale are copper and nickel. Fundamental observations on the subject of activation and chemical metallization are to be found, e.g., in "Kunststoff-Galvanisierung" [Electroplating of plastic] (Leuze Verlag, Saulgau/Wurtt.). Similar prior art regarding the activation and metallization of porous, non-conductive plastic substrate sheets can also be gathered from the German Patents 3,631,055, 3,837,835, 3,637,130, 3,710,895, 3,925,232 and 4,106,696.
Carrying out the conventional process has disadvantageously resulted in the spent and/or excess activating solution hydroextracted from the substrate sheets after the activation operation, and the amounts of water employed, after activation and/or metallization have been carried out, for rinsing and washing all being rejected as effluent. That is, as a rule the solution and/or water is used only once. This has resulted in high costs for fresh batches of activating solutions and metallizing solutions each time and for the rinsing and/or washing water required after the substrate sheets had been activated or metallized. The amounts of effluent are many times the pore volume of the substrate sheets treated. In addition, the environment has also been polluted indirectly by these effluents.
The hydroextraction and/or rinsing water, contaminated with palladium/tin particles, after the activation operation could not be used for making up a metallizing solution, as the solution formed autodecomposes catalytically when the reaction partners are added. In the event of employing this effluent for making up a new activating or hydrolysis solution, it was believed that the precipitated palladium/tin particles would have a flocculent effect on the newly generated palladium/tin particles to be kept in solution as a complex or colloidally.
Nor was the spent and/or excess metallizing solution obtained upon rinsing after the metallization operation deemed suitable for being reprocessed into a fresh metallizing solution. When plastic substrates are nickel-plated chemically, nickel flitters are flushed out which are found not only in the hydroextracted nickel-plating solution but also in the subsequently hydroextracted rinsing water. These rinsed-off metal flitters autocatalytically decompose a fresh metallizing solution.
There are known methods of reducing the amounts of effluent produced in the course of activation and metallization of plastic substrate sheets. The purpose of such methods is, firstly, to reduce expenditure for the water employed and, secondly, to reduce the load which the effluent produced presents to the environment. Thus, for example, German Patent 3,637,130, describes that a textile material, after it has been activated, is immediately brought into contact with a metallizing solution, without an intermediary rinsing operation. German Patent 4,033,518 discloses a process in which the effluent produced during the activation of plastic substrate sheets is reprocessed. German Patent 3,925,232 relates to a process for the chemical metallization of activated, porous plastic substrates, in which process the volume of the chemical metallizing solution is chosen to be smaller than the free pore volume of the porous substrate. German Patent 4,106,696 is based on a process for producing chemically metallized porous plastic substrate sheets, requiring as few individual steps as possible and also producing as little effluent as possible.
These known methods achieve only a small reduction in the amounts of effluent produced. In view of the costs of the make-up water employed in the manufacture of activated and metallized plastic substrate sheets and of the environmental pollution by the effluent produced, these methods cannot be regarded as satisfactory.
An object of the present invention is, therefore, to overcome the drawbacks of known methods and to provide a process which makes it possible, in the process of metallizing plastic substrate sheets, to reuse a number of times, after appropriate processing, make-up water employed in carrying out the process, for the individual process steps and thus to reduce by a greater extent the amount of effluent produced.
This object has been achieved, according to the present invention, by a method in which the separated metallizing solution and the rinsing water after use thereof are each collected separately and subjected to a treatment for reuse, the collected, used metallizing solution is set to an approximately neutral pH and is distilled, with distillate being reused as a second rinsing water, and a reusable metallizing solution is prepared from a first rinsing water by adding the chemical metallizing components.
In practice, the process is carried out such that the effluent produced in the first rinsing and washing operation, after an activated plastic substrate sheet has been metallized, is collected. This "effluent" is then used, by adding the individual components, to prepare once again a chemical metallizing solution by way of which in a similar manner further activated plastic substrate sheets are metallized. The spent and/or excess metallizing solutions hydroextracted from the substrate sheets after metallization has been carried out are in turn collected. This solution is then set, with a weak acid, to a slightly acidic pH (from 4 to 5). Advantageously, the pH is set by employing acidic, spent and/or excess activating solution which after the activation operation of the substrate sheet has been hydroextracted therefrom and which is normally slightly acidic owing to mineral acids. This again reduces the amount of effluent produced.
The collected metallizing solution set to this pH is then distilled off in a distillation operation (e.g. in a rotary evaporator) at approximately from 70.degree. to 80.degree. C. This distillation is carried out until the distillate is approximately neutral (pH from 6 to 8), and the distillate obtained is then again used as rinsing and/or washing water for substrate sheets after these have been metallized. In the distillation step, the pollutants present in the spent activating and metallizing solutions are left behind in the distillation appliance as a residue in the form of a sludge. During distillation, care must of course be taken to ensure that the substances settling in the sludge of the evaporation appliance, which are thermally decomposable even at low temperatures, are not decomposed.
The non-conductive, large-area plastic substrates employed in carrying out the process are preferably sheets of nonwoven, needlefelt or open-pore foamed materials consisting of polyethylene or polypropylene, and, in the initial state resulting from their manufacture, have a porosity of greater than 50%.